Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods for seismic data acquisition using clustered seismic vibrators that are placed at different vibration points within a predetermined distance from one another and are activated simultaneously.
Discussion of the Background
In the oil and gas industry sector particularly, seismic surveys are commonly used to search for and evaluate subterranean hydrocarbon deposits. Seismic surveys are exploration methods that gather and record data related to seismic wave reflections from interfaces between geological layers. Seismic data is then used to create detailed models of the underlying geological structure.
FIG. 1 schematically depicts a land seismic survey system 100 for seismic exploration in a land environment. System 100 includes a source 110 (e.g., a vibratory source) that generates seismic waves, receivers 120 (e.g., geophones) for detecting the seismic reflections, and a recorder 130. Recorder 130 is configured to record electrical signals or seismic data resulting from sampling electrical signals generated by receivers 120 upon detecting seismic reflections. To perform the seismic survey, source 110, receivers 120 and recorder 130 are positioned on ground surface 150. However, source 110 and recorder 130, being carried on trucks, may be repositioned, while receivers 120 are usually arranged over the surveyed geological structure along receiver lines.
In operation, source 110 generates seismic waves that may include a surface waves 140 and body waves 160 that may be partially reflected at an interface 170 between two geological layers inside which the seismic waves propagate with different speeds. Each receiver 120 receives the full wavefield (i.e., both surface and body waves) and converts it into an electrical signal.
Source 110 may be a vibratory source. Vibratory sources are actuated in sweeps lasting generally between 2 and 60 seconds, during which time the vibratory sources generate seismic waves whose frequencies vary over a given range (e.g., 2-200 Hz). Receivers 120 record data for a period during and after the sweep time has ended. This period after the sweep time is known as listening time. During listening time, source 110 may be moved from one location to a next location according to a predetermined survey plan. The locations where seismic sources are activated are known as vibration points.
FIG. 2 illustrates a conventional land survey plan. Seismic receivers 210 (only a few are labeled) are arranged along receiver lines 220, 224, 226. Individual seismic receivers may be deployed along the receiver lines at substantially equal intervals, for example, of about 25 m. Distance between adjacent receiver lines (e.g., 220 to 224, or 224 to 226) may be, for example, about 200 m. The receiver arrangement illustrated in FIG. 2 may be achieved when the surveyed area allows such a layout. However, in practice, the lines may not be parallel straight lines.
Trucks carrying seismic sources 230-241 move along shot lines SL, stopping at successive vibration points. FIG. 2 illustrates a grid of vibration points at intersections of shot lines SL (parallel to receiver lines 220, 224, 226) and lines SN perpendicular to the shot lines SL. A distance between adjacent shot lines may be about 25 m, and a distance between lines SN may be about 50 m. The trucks may use Global Positioning System (GPS) equipment to position the seismic sources at the vibration points according to the survey plan. After a source generates seismic waves according to a predetermined sweep at a vibration point, the truck moves to a next vibration point. Assuming the sources on trucks 230-241 are actuated one after another, a truck has about ¾ minutes to move to a new vibration point. At least one source is ready to be activated at a vibration point when a previous sweep's listening time ends.
A shooting plan includes a large number of vibration points to acquire the information necessary to arrive at conclusions related to the presence and location of subterranean hydrocarbon deposits. Even if the combined sweep and listening time for each one of the vibration points is less than 30 s, acquiring seismic data for all vibration points in a typical survey plan takes a long time, which is expensive in terms of equipment and personnel. Meanwhile, there is a permanent desire to increase vibration points' density to achieve better images of the explored underground structures. Therefore, increasing productivity is an ongoing effort.
One method of increasing productivity of acquiring seismic data known as “slip-sweep” is described in the publication, Wams, J. and Rozemond J. (1998), “Recent Developments in 3-D acquisition using vibroseis in Oman”, Leading Edge 17 No. 8, pp. 1053-1063, the content of which is incorporated herein by reference. According to the slip-sweep method, a next sweep starts before the end of listening time related to a previous sweep. The time interval between successive sweeps is known as slip time. However, to be able to separate seismic data corresponding to the different sweeps, slip time is limited by the requirement to avoid overlapping harmonic energy generated during successive sweeps. For example, using the slip-sweep method for a slip time of 5 s with the plan in FIG. 2, a nominal productivity of 720 surveyed points per hour may theoretically be achieved (in practice productivity is only about 600 surveyed points per hour). To separate seismic data corresponding to seismic vibrations produced by different sources, methods described in U.S. Pat. No. 7,050,356 request acquiring data by repeatedly sweeping at the same vibration points with different slip times. In this case, it appears the productivity gained by starting a next sweep before a previous sweep ends is offset by repeating the sweep multiple times.
Some harmonic noise reduction algorithms have been developed, for example, as described in U.S. Pat. No. 6,603,707 and U.S. Pat. No. 8,451,686, the contents of which are incorporated herein by reference. Using these harmonic noise reduction algorithms has allowed productivity to further increase.
Another technique of increasing seismic data acquisition productivity known as “independent simultaneous sweeping” (ISS) is described in U.S. Patent Application Publication No. 2012/0290213, the content of which is incorporated herein by reference. ISS uses statistical methods to remove interference due to unsynchronized overlapping sweeps of sources located at large distances there-between (e.g., 2 km).
However, as ISS separation remains imperfect for a large number of closely spaced sources it remains desirable to enhance the statistical source separation by using acquisition schemes specially designed for separation.